// © 2016 and later: Unicode, Inc. and others.
// License & terms of use: http://www.unicode.org/copyright.html
/*
 *******************************************************************************
 * Copyright (C) 1996-2016, International Business Machines Corporation and
 * others. All Rights Reserved.
 *******************************************************************************
 */
package com.ibm.icu.text;

import com.ibm.icu.impl.ICUData;
import com.ibm.icu.impl.ICUResourceBundle;
import com.ibm.icu.impl.Utility;
import com.ibm.icu.impl.UtilityExtensions;
import com.ibm.icu.text.RuleBasedTransliterator.Data;
import com.ibm.icu.text.TransliteratorIDParser.SingleID;
import com.ibm.icu.util.CaseInsensitiveString;
import com.ibm.icu.util.ULocale;
import com.ibm.icu.util.ULocale.Category;
import com.ibm.icu.util.UResourceBundle;
import java.text.MessageFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.MissingResourceException;
import java.util.Objects;
import java.util.function.Supplier;

/**
 * <code>Transliterator</code> is an abstract class that transliterates text from one format to
 * another. The most common kind of transliterator is a script, or alphabet, transliterator. For
 * example, a Russian to Latin transliterator changes Russian text written in Cyrillic characters to
 * phonetically equivalent Latin characters. It does not <em>translate</em> Russian to English!
 * Transliteration, unlike translation, operates on characters, without reference to the meanings of
 * words and sentences.
 *
 * <p>Although script conversion is its most common use, a transliterator can actually perform a
 * more general class of tasks. In fact, <code>Transliterator</code> defines a very general API
 * which specifies only that a segment of the input text is replaced by new text. The particulars of
 * this conversion are determined entirely by subclasses of <code>Transliterator</code>.
 *
 * <p><b>Transliterators are stateless</b>
 *
 * <p><code>Transliterator</code> objects are <em>stateless</em>; they retain no information between
 * calls to <code>transliterate()</code>. As a result, threads may share transliterators without
 * synchronizing them. This might seem to limit the complexity of the transliteration operation. In
 * practice, subclasses perform complex transliterations by delaying the replacement of text until
 * it is known that no other replacements are possible. In other words, although the <code>
 * Transliterator</code> objects are stateless, the source text itself embodies all the needed
 * information, and delayed operation allows arbitrary complexity.
 *
 * <p><b>Batch transliteration</b>
 *
 * <p>The simplest way to perform transliteration is all at once, on a string of existing text. This
 * is referred to as <em>batch</em> transliteration. For example, given a string <code>input</code>
 * and a transliterator <code>t</code>, the call
 *
 * <blockquote>
 *
 * <code>String result = t.transliterate(input);
 * </code>
 *
 * </blockquote>
 *
 * will transliterate it and return the result. Other methods allow the client to specify a
 * substring to be transliterated and to use {@link Replaceable} objects instead of strings, in
 * order to preserve out-of-band information (such as text styles).
 *
 * <p><b>Keyboard transliteration</b>
 *
 * <p>Somewhat more involved is <em>keyboard</em>, or incremental transliteration. This is the
 * transliteration of text that is arriving from some source (typically the user's keyboard) one
 * character at a time, or in some other piecemeal fashion.
 *
 * <p>In keyboard transliteration, a <code>Replaceable</code> buffer stores the text. As text is
 * inserted, as much as possible is transliterated on the fly. This means a GUI that displays the
 * contents of the buffer may show text being modified as each new character arrives.
 *
 * <p>Consider the simple rule-based Transliterator:
 *
 * <blockquote>
 *
 * <code>
 * th&gt;{theta}<br>
 * t&gt;{tau}
 * </code>
 *
 * </blockquote>
 *
 * When the user types 't', nothing will happen, since the transliterator is waiting to see if the
 * next character is 'h'. To remedy this, we introduce the notion of a cursor, marked by a '|' in
 * the output string:
 *
 * <blockquote>
 *
 * <code>
 * t&gt;|{tau}<br>
 * {tau}h&gt;{theta}
 * </code>
 *
 * </blockquote>
 *
 * Now when the user types 't', tau appears, and if the next character is 'h', the tau changes to a
 * theta. This is accomplished by maintaining a cursor position (independent of the insertion point,
 * and invisible in the GUI) across calls to <code>transliterate()</code>. Typically, the cursor
 * will be coincident with the insertion point, but in a case like the one above, it will precede
 * the insertion point.
 *
 * <p>Keyboard transliteration methods maintain a set of three indices that are updated with each
 * call to <code>transliterate()</code>, including the cursor, start, and limit. These indices are
 * changed by the method, and they are passed in and out via a Position object. The <code>start
 * </code> index marks the beginning of the substring that the transliterator will look at. It is
 * advanced as text becomes committed (but it is not the committed index; that's the <code>cursor
 * </code>). The <code>cursor</code> index, described above, marks the point at which the
 * transliterator last stopped, either because it reached the end, or because it required more
 * characters to disambiguate between possible inputs. The <code>cursor</code> can also be
 * explicitly set by rules. Any characters before the <code>cursor</code> index are frozen; future
 * keyboard transliteration calls within this input sequence will not change them. New text is
 * inserted at the <code>limit</code> index, which marks the end of the substring that the
 * transliterator looks at.
 *
 * <p>Because keyboard transliteration assumes that more characters are to arrive, it is
 * conservative in its operation. It only transliterates when it can do so unambiguously. Otherwise
 * it waits for more characters to arrive. When the client code knows that no more characters are
 * forthcoming, perhaps because the user has performed some input termination operation, then it
 * should call <code>finishTransliteration()</code> to complete any pending transliterations.
 *
 * <p><b>Inverses</b>
 *
 * <p>Pairs of transliterators may be inverses of one another. For example, if transliterator
 * <b>A</b> transliterates characters by incrementing their Unicode value (so "abc" -&gt; "def"),
 * and transliterator <b>B</b> decrements character values, then <b>A</b> is an inverse of <b>B</b>
 * and vice versa. If we compose <b>A</b> with <b>B</b> in a compound transliterator, the result is
 * the identity transliterator, that is, a transliterator that does not change its input text.
 *
 * <p>The <code>Transliterator</code> method <code>getInverse()</code> returns a transliterator's
 * inverse, if one exists, or <code>null</code> otherwise. However, the result of <code>getInverse()
 * </code> usually will <em>not</em> be a true mathematical inverse. This is because true inverse
 * transliterators are difficult to formulate. For example, consider two transliterators: <b>AB</b>,
 * which transliterates the character 'A' to 'B', and <b>BA</b>, which transliterates 'B' to 'A'. It
 * might seem that these are exact inverses, since
 *
 * <blockquote>
 *
 * "A" x <b>AB</b> -&gt; "B"<br>
 * "B" x <b>BA</b> -&gt; "A"
 *
 * </blockquote>
 *
 * where 'x' represents transliteration. However,
 *
 * <blockquote>
 *
 * "ABCD" x <b>AB</b> -&gt; "BBCD"<br>
 * "BBCD" x <b>BA</b> -&gt; "AACD"
 *
 * </blockquote>
 *
 * so <b>AB</b> composed with <b>BA</b> is not the identity. Nonetheless, <b>BA</b> may be usefully
 * considered to be <b>AB</b>'s inverse, and it is on this basis that <b>AB</b><code>.getInverse()
 * </code> could legitimately return <b>BA</b>.
 *
 * <p><b>Filtering</b>
 *
 * <p>Each transliterator has a filter, which restricts changes to those characters selected by the
 * filter. The filter affects just the characters that are changed -- the characters outside of the
 * filter are still part of the context for the filter. For example, in the following even though
 * 'x' is filtered out, and doesn't convert to y, it does affect the conversion of 'a'.
 *
 * <pre>
 * String rules = &quot;x &gt; y; x{a} &gt; b; &quot;;
 * Transliterator tempTrans = Transliterator.createFromRules(&quot;temp&quot;, rules, Transliterator.FORWARD);
 * tempTrans.setFilter(new UnicodeSet(&quot;[a]&quot;));
 * String tempResult = tempTrans.transform(&quot;xa&quot;);
 * // results in &quot;xb&quot;
 * </pre>
 *
 * <p><b>IDs and display names</b>
 *
 * <p>A transliterator is designated by a short identifier string or <em>ID</em>. IDs follow the
 * format <em>source-destination</em>, where <em>source</em> describes the entity being replaced,
 * and <em>destination</em> describes the entity replacing <em>source</em>. The entities may be the
 * names of scripts, particular sequences of characters, or whatever else it is that the
 * transliterator converts to or from. For example, a transliterator from Russian to Latin might be
 * named "Russian-Latin". A transliterator from keyboard escape sequences to Latin-1 characters
 * might be named "KeyboardEscape-Latin1". By convention, system entity names are in English, with
 * the initial letters of words capitalized; user entity names may follow any format so long as they
 * do not contain dashes.
 *
 * <p>In addition to programmatic IDs, transliterator objects have display names for presentation in
 * user interfaces, returned by {@link #getDisplayName}.
 *
 * <p><b>Factory methods and registration</b>
 *
 * <p>In general, client code should use the factory method <code>getInstance()</code> to obtain an
 * instance of a transliterator given its ID. Valid IDs may be enumerated using <code>
 * getAvailableIDs()</code>. Since transliterators are stateless, multiple calls to <code>
 * getInstance()</code> with the same ID will return the same object.
 *
 * <p>In addition to the system transliterators registered at startup, user transliterators may be
 * registered by calling <code>registerInstance()</code> at run time. To register a transliterator
 * subclass without instantiating it (until it is needed), users may call <code>registerClass()
 * </code>.
 *
 * <p><b>Composed transliterators</b>
 *
 * <p>In addition to built-in system transliterators like "Latin-Greek", there are also built-in
 * <em>composed</em> transliterators. These are implemented by composing two or more component
 * transliterators. For example, if we have scripts "A", "B", "C", and "D", and we want to
 * transliterate between all pairs of them, then we need to write 12 transliterators: "A-B", "A-C",
 * "A-D", "B-A",..., "D-A", "D-B", "D-C". If it is possible to convert all scripts to an
 * intermediate script "M", then instead of writing 12 rule sets, we only need to write 8: "A~M",
 * "B~M", "C~M", "D~M", "M~A", "M~B", "M~C", "M~D". (This might not seem like a big win, but it's
 * really 2<em>n</em> vs. <em>n</em> <sup>2</sup> - <em>n</em>, so as <em>n</em> gets larger the
 * gain becomes significant. With 9 scripts, it's 18 vs. 72 rule sets, a big difference.) Note the
 * use of "~" rather than "-" for the script separator here; this indicates that the given
 * transliterator is intended to be composed with others, rather than be used as is.
 *
 * <p>Composed transliterators can be instantiated as usual. For example, the system transliterator
 * "Devanagari-Gujarati" is a composed transliterator built internally as
 * "Devanagari~InterIndic;InterIndic~Gujarati". When this transliterator is instantiated, it appears
 * externally to be a standard transliterator (e.g., getID() returns "Devanagari-Gujarati").
 *
 * <p><b>Subclassing</b>
 *
 * <p>Subclasses must implement the abstract method <code>handleTransliterate()</code>.
 *
 * <p>Subclasses should override the <code>transliterate()</code> method taking a <code>Replaceable
 * </code> and the <code>transliterate()</code> method taking a <code>String</code> and <code>
 * StringBuffer</code> if the performance of these methods can be improved over the performance
 * obtained by the default implementations in this class.
 *
 * <p><b>Rule syntax</b>
 *
 * <p>A set of rules determines how to perform translations. Rules within a rule set are separated
 * by semicolons (';'). To include a literal semicolon, prefix it with a backslash ('\'). Unicode
 * Pattern_White_Space is ignored. If the first non-blank character on a line is '#', the entire
 * line is ignored as a comment.
 *
 * <p>Each set of rules consists of two groups, one forward, and one reverse. This is a convention
 * that is not enforced; rules for one direction may be omitted, with the result that translations
 * in that direction will not modify the source text. In addition, bidirectional forward-reverse
 * rules may be specified for symmetrical transformations.
 *
 * <p>Note: Another description of the Transliterator rule syntax is available in <a
 * href="https://www.unicode.org/reports/tr35/tr35-general.html#Transform_Rules_Syntax">section
 * Transform Rules Syntax of UTS #35: Unicode LDML</a>. The rules are shown there using arrow
 * symbols ← and → and ↔. ICU supports both those and the equivalent ASCII symbols &lt; and &gt; and
 * &lt;&gt;.
 *
 * <p>Rule statements take one of the following forms:
 *
 * <dl>
 *   <dt><code>$alefmadda=\\u0622;</code>
 *   <dd><strong>Variable definition.</strong> The name on the left is assigned the text on the
 *       right. In this example, after this statement, instances of the left hand name, &quot;<code>
 *       $alefmadda</code>&quot;, will be replaced by the Unicode character U+0622. Variable names
 *       must begin with a letter and consist only of letters, digits, and underscores. Case is
 *       significant. Duplicate names cause an exception to be thrown, that is, variables cannot be
 *       redefined. The right hand side may contain well-formed text of any length, including no
 *       text at all (&quot;<code>$empty=;</code>&quot;). The right hand side may contain embedded
 *       <code>UnicodeSet</code> patterns, for example, &quot;<code>$softvowel=[eiyEIY]</code>
 *       &quot;.
 *   <dt><code>ai&gt;$alefmadda;</code>
 *   <dd><strong>Forward translation rule.</strong> This rule states that the string on the left
 *       will be changed to the string on the right when performing forward transliteration.
 *   <dt><code>ai&lt;$alefmadda;</code>
 *   <dd><strong>Reverse translation rule.</strong> This rule states that the string on the right
 *       will be changed to the string on the left when performing reverse transliteration.
 * </dl>
 *
 * <dl>
 *   <dt><code>ai&lt;&gt;$alefmadda;</code>
 *   <dd><strong>Bidirectional translation rule.</strong> This rule states that the string on the
 *       right will be changed to the string on the left when performing forward transliteration,
 *       and vice versa when performing reverse transliteration.
 * </dl>
 *
 * <p>Translation rules consist of a <em>match pattern</em> and an <em>output string</em>. The match
 * pattern consists of literal characters, optionally preceded by context, and optionally followed
 * by context. Context characters, like literal pattern characters, must be matched in the text
 * being transliterated. However, unlike literal pattern characters, they are not replaced by the
 * output text. For example, the pattern &quot;<code>abc{def}</code>&quot; indicates the characters
 * &quot;<code>def</code>&quot; must be preceded by &quot;<code>abc</code>&quot; for a successful
 * match. If there is a successful match, &quot;<code>def</code>&quot; will be replaced, but not
 * &quot;<code>abc</code>&quot;. The final '<code>}</code>' is optional, so &quot;<code>abc{def
 * </code>&quot; is equivalent to &quot;<code>abc{def}</code>&quot;. Another example is &quot;<code>
 * {123}456</code>&quot; (or &quot;<code>123}456</code>&quot;) in which the literal pattern &quot;
 * <code>123</code>&quot; must be followed by &quot;<code>456</code>&quot;.
 *
 * <p>The output string of a forward or reverse rule consists of characters to replace the literal
 * pattern characters. If the output string contains the character '<code>|</code>', this is taken
 * to indicate the location of the <em>cursor</em> after replacement. The cursor is the point in the
 * text at which the next replacement, if any, will be applied. The cursor is usually placed within
 * the replacement text; however, it can actually be placed into the precending or following context
 * by using the special character '@'. Examples:
 *
 * <pre>
 *     a {foo} z &gt; | @ bar; # foo -&gt; bar, move cursor before a
 *     {foo} xyz &gt; bar @@|; #&nbsp;foo -&gt; bar, cursor between y and z
 * </pre>
 *
 * <p><b>UnicodeSet</b>
 *
 * <p><code>UnicodeSet</code> patterns may appear anywhere that makes sense. They may appear in
 * variable definitions. Contrariwise, <code>UnicodeSet</code> patterns may themselves contain
 * variable references, such as &quot;<code>$a=[a-z];$not_a=[^$a]</code>&quot;, or &quot;<code>
 * $range=a-z;$ll=[$range]</code>&quot;.
 *
 * <p><code>UnicodeSet</code> patterns may also be embedded directly into rule strings. Thus, the
 * following two rules are equivalent:
 *
 * <pre>
 *     $vowel=[aeiou]; $vowel&gt;'*'; # One way to do this
 *     [aeiou]&gt;'*'; # Another way
 * </pre>
 *
 * <p>See {@link UnicodeSet} for more documentation and examples.
 *
 * <p><b>Segments</b>
 *
 * <p>Segments of the input string can be matched and copied to the output string. This makes
 * certain sets of rules simpler and more general, and makes reordering possible. For example:
 *
 * <pre>
 *     ([a-z]) &gt; $1 $1; # double lowercase letters
 *     ([:Lu:]) ([:Ll:]) &gt; $2 $1; # reverse order of Lu-Ll pairs
 * </pre>
 *
 * <p>The segment of the input string to be copied is delimited by &quot;<code>(</code>&quot; and
 * &quot;<code>)</code>&quot;. Up to nine segments may be defined. Segments may not overlap. In the
 * output string, &quot;<code>$1</code>&quot; through &quot;<code>$9</code>&quot; represent the
 * input string segments, in left-to-right order of definition.
 *
 * <p><b>Anchors</b>
 *
 * <p>Patterns can be anchored to the beginning or the end of the text. This is done with the
 * special characters '<code>^</code>' and '<code>$</code>'. For example:
 *
 * <pre>
 *   ^ a&nbsp;&nbsp; &gt; 'BEG_A'; &nbsp;&nbsp;# match 'a' at start of text
 *   &nbsp; a&nbsp;&nbsp; &gt; 'A'; # match other instances of 'a'
 *   &nbsp; z $ &gt; 'END_Z'; &nbsp;&nbsp;# match 'z' at end of text
 *   &nbsp; z&nbsp;&nbsp; &gt; 'Z';&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; # match other instances of 'z'
 * </pre>
 *
 * <p>It is also possible to match the beginning or the end of the text using a <code>UnicodeSet
 * </code>. This is done by including a virtual anchor character '<code>$</code>' at the end of the
 * set pattern. Although this is usually the match character for the end anchor, the set will match
 * either the beginning or the end of the text, depending on its placement. For example:
 *
 * <pre>
 *   $x = [a-z$]; &nbsp;&nbsp;# match 'a' through 'z' OR anchor
 *   $x 1&nbsp;&nbsp;&nbsp; &gt; 2;&nbsp;&nbsp; # match '1' after a-z or at the start
 *   &nbsp;&nbsp; 3 $x &gt; 4; &nbsp;&nbsp;# match '3' before a-z or at the end
 * </pre>
 *
 * <p><b>Example</b>
 *
 * <p>The following example rules illustrate many of the features of the rule language.
 *
 * <table style="border:none;padding:4px;">
 *     <tr>
 *         <td style="vertical-align: top;">Rule 1.</td>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>abc{def}&gt;x|y</code></td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top;">Rule 2.</td>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>xyz&gt;r</code></td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top;">Rule 3.</td>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>yz&gt;q</code></td>
 *     </tr>
 * </table>
 *
 * <p>Applying these rules to the string &quot;<code>adefabcdefz</code>&quot; yields the following
 * results:
 *
 * <table style="border:none;padding:4px;">
 *     <tr>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>|adefabcdefz</code></td>
 *         <td style="vertical-align: top;">Initial state, no rules match. Advance
 *         cursor.</td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>a|defabcdefz</code></td>
 *         <td style="vertical-align: top;">Still no match. Rule 1 does not match
 *         because the preceding context is not present.</td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>ad|efabcdefz</code></td>
 *         <td style="vertical-align: top;">Still no match. Keep advancing until
 *         there is a match...</td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>ade|fabcdefz</code></td>
 *         <td style="vertical-align: top;">...</td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>adef|abcdefz</code></td>
 *         <td style="vertical-align: top;">...</td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>adefa|bcdefz</code></td>
 *         <td style="vertical-align: top;">...</td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>adefab|cdefz</code></td>
 *         <td style="vertical-align: top;">...</td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>adefabc|defz</code></td>
 *         <td style="vertical-align: top;">Rule 1 matches; replace &quot;<code>def</code>&quot;
 *         with &quot;<code>xy</code>&quot; and back up the cursor
 *         to before the '<code>y</code>'.</td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>adefabcx|yz</code></td>
 *         <td style="vertical-align: top;">Although &quot;<code>xyz</code>&quot; is
 *         present, rule 2 does not match because the cursor is
 *         before the '<code>y</code>', not before the '<code>x</code>'.
 *         Rule 3 does match. Replace &quot;<code>yz</code>&quot;
 *         with &quot;<code>q</code>&quot;.</td>
 *     </tr>
 *     <tr>
 *         <td style="vertical-align: top; write-space: nowrap;"><code>adefabcxq|</code></td>
 *         <td style="vertical-align: top;">The cursor is at the end;
 *         transliteration is complete.</td>
 *     </tr>
 * </table>
 *
 * <p>The order of rules is significant. If multiple rules may match at some point, the first
 * matching rule is applied.
 *
 * <p>Forward and reverse rules may have an empty output string. Otherwise, an empty left or right
 * hand side of any statement is a syntax error.
 *
 * <p>Single quotes are used to quote any character other than a digit or letter. To specify a
 * single quote itself, inside or outside of quotes, use two single quotes in a row. For example,
 * the rule &quot;<code>'&gt;'&gt;o''clock</code>&quot; changes the string &quot;<code>&gt;</code>
 * &quot; to the string &quot;<code>o'clock</code>&quot;.
 *
 * <p><b>Notes</b>
 *
 * <p>While a Transliterator is being built from rules, it checks that the rules are added in proper
 * order. For example, if the rule &quot;a&gt;x&quot; is followed by the rule &quot;ab&gt;y&quot;,
 * then the second rule will throw an exception. The reason is that the second rule can never be
 * triggered, since the first rule always matches anything it matches. In other words, the first
 * rule <em>masks</em> the second rule.
 *
 * @author Alan Liu
 * @stable ICU 2.0
 */
public abstract class Transliterator implements StringTransform {
    /**
     * Direction constant indicating the forward direction in a transliterator, e.g., the forward
     * rules of a rule-based Transliterator. An "A-B" transliterator transliterates A to B when
     * operating in the forward direction, and B to A when operating in the reverse direction.
     *
     * @stable ICU 2.0
     */
    public static final int FORWARD = 0;

    /**
     * Direction constant indicating the reverse direction in a transliterator, e.g., the reverse
     * rules of a rule-based Transliterator. An "A-B" transliterator transliterates A to B when
     * operating in the forward direction, and B to A when operating in the reverse direction.
     *
     * @stable ICU 2.0
     */
    public static final int REVERSE = 1;

    /**
     * Position structure for incremental transliteration. This data structure defines two
     * substrings of the text being transliterated. The first region, [contextStart, contextLimit),
     * defines what characters the transliterator will read as context. The second region, [start,
     * limit), defines what characters will actually be transliterated. The second region should be
     * a subset of the first.
     *
     * <p>After a transliteration operation, some of the indices in this structure will be modified.
     * See the field descriptions for details.
     *
     * <p>contextStart &lt;= start &lt;= limit &lt;= contextLimit
     *
     * <p>Note: All index values in this structure must be at code point boundaries. That is, none
     * of them may occur between two code units of a surrogate pair. If any index does split a
     * surrogate pair, results are unspecified.
     *
     * @stable ICU 2.0
     */
    public static class Position {

        /**
         * Beginning index, inclusive, of the context to be considered for a transliteration
         * operation. The transliterator will ignore anything before this index. INPUT/OUTPUT
         * parameter: This parameter is updated by a transliteration operation to reflect the
         * maximum amount of antecontext needed by a transliterator.
         *
         * @stable ICU 2.0
         */
        public int contextStart;

        /**
         * Ending index, exclusive, of the context to be considered for a transliteration operation.
         * The transliterator will ignore anything at or after this index. INPUT/OUTPUT parameter:
         * This parameter is updated to reflect changes in the length of the text, but points to the
         * same logical position in the text.
         *
         * @stable ICU 2.0
         */
        public int contextLimit;

        /**
         * Beginning index, inclusive, of the text to be transliteratd. INPUT/OUTPUT parameter: This
         * parameter is advanced past characters that have already been transliterated by a
         * transliteration operation.
         *
         * @stable ICU 2.0
         */
        public int start;

        /**
         * Ending index, exclusive, of the text to be transliteratd. INPUT/OUTPUT parameter: This
         * parameter is updated to reflect changes in the length of the text, but points to the same
         * logical position in the text.
         *
         * @stable ICU 2.0
         */
        public int limit;

        /**
         * Constructs a Position object with start, limit, contextStart, and contextLimit all equal
         * to zero.
         *
         * @stable ICU 2.0
         */
        public Position() {
            this(0, 0, 0, 0);
        }

        /**
         * Constructs a Position object with the given start, contextStart, and contextLimit. The
         * limit is set to the contextLimit.
         *
         * @stable ICU 2.0
         */
        public Position(int contextStart, int contextLimit, int start) {
            this(contextStart, contextLimit, start, contextLimit);
        }

        /**
         * Constructs a Position object with the given start, limit, contextStart, and contextLimit.
         *
         * @stable ICU 2.0
         */
        public Position(int contextStart, int contextLimit, int start, int limit) {
            this.contextStart = contextStart;
            this.contextLimit = contextLimit;
            this.start = start;
            this.limit = limit;
        }

        /**
         * Constructs a Position object that is a copy of another.
         *
         * @stable ICU 2.6
         */
        public Position(Position pos) {
            set(pos);
        }

        /**
         * Copies the indices of this position from another.
         *
         * @stable ICU 2.6
         */
        public void set(Position pos) {
            contextStart = pos.contextStart;
            contextLimit = pos.contextLimit;
            start = pos.start;
            limit = pos.limit;
        }

        /**
         * Returns true if this Position is equal to the given object.
         *
         * @stable ICU 2.0
         */
        @Override
        public boolean equals(Object obj) {
            if (obj instanceof Position) {
                Position pos = (Position) obj;
                return contextStart == pos.contextStart
                        && contextLimit == pos.contextLimit
                        && start == pos.start
                        && limit == pos.limit;
            }
            return false;
        }

        /**
         * {@inheritDoc}
         *
         * @stable ICU 2.0
         */
        @Override
        public int hashCode() {
            return Objects.hash(contextStart, contextLimit, start, limit);
        }

        /**
         * Returns a string representation of this Position.
         *
         * @return a string representation of the object.
         * @stable ICU 2.0
         */
        @Override
        public String toString() {
            return "[cs="
                    + contextStart
                    + ", s="
                    + start
                    + ", l="
                    + limit
                    + ", cl="
                    + contextLimit
                    + "]";
        }

        /**
         * Check all bounds. If they are invalid, throw an exception.
         *
         * @param length the length of the string this object applies to
         * @exception IllegalArgumentException if any indices are out of bounds
         * @stable ICU 2.0
         */
        public final void validate(int length) {
            if (contextStart < 0
                    || start < contextStart
                    || limit < start
                    || contextLimit < limit
                    || length < contextLimit) {
                throw new IllegalArgumentException(
                        "Invalid Position {cs="
                                + contextStart
                                + ", s="
                                + start
                                + ", l="
                                + limit
                                + ", cl="
                                + contextLimit
                                + "}, len="
                                + length);
            }
        }
    }

    /** Programmatic name, e.g., "Latin-Arabic". */
    private String ID;

    /**
     * This transliterator's filter. Any character for which {@code filter.contains()} returns
     * {@code false} will not be altered by this transliterator. If {@code filter} is {@code null}
     * then no filtering is applied.
     */
    private UnicodeSet filter;

    private int maximumContextLength = 0;

    /** System transliterator registry. */
    private static TransliteratorRegistry registry;

    private static Map<CaseInsensitiveString, String> displayNameCache;

    /**
     * Prefix for resource bundle key for the display name for a transliterator. The ID is appended
     * to this to form the key. The resource bundle value should be a String.
     */
    private static final String RB_DISPLAY_NAME_PREFIX = "%Translit%%";

    /**
     * Prefix for resource bundle key for the display name for a transliterator SCRIPT. The ID is
     * appended to this to form the key. The resource bundle value should be a String.
     */
    private static final String RB_SCRIPT_DISPLAY_NAME_PREFIX = "%Translit%";

    /**
     * Resource bundle key for display name pattern. The resource bundle value should be a String
     * forming a MessageFormat pattern, e.g.: "{0,choice,0#|1#{1} Transliterator|2#{1} to {2}
     * Transliterator}".
     */
    private static final String RB_DISPLAY_NAME_PATTERN = "TransliteratorNamePattern";

    /** Delimiter between elements in a compound ID. */
    static final char ID_DELIM = ';';

    /** Delimiter before target in an ID. */
    static final char ID_SEP = '-';

    /** Delimiter before variant in an ID. */
    static final char VARIANT_SEP = '/';

    /**
     * To enable debugging output in the Transliterator component, set DEBUG to true.
     *
     * <p>N.B. Make sure to recompile all of the com.ibm.icu.text package after changing this.
     * Easiest way to do this is 'ant clean core' ('ant' will NOT pick up the dependency
     * automatically).
     *
     * <p><<This generates a lot of output.>>
     */
    static final boolean DEBUG = false;

    /**
     * Default constructor.
     *
     * @param ID the string identifier for this transliterator
     * @param filter the filter. Any character for which {@code filter.contains()} returns {@code
     *     false} will not be altered by this transliterator. If {@code filter} is {@code null} then
     *     no filtering is applied.
     * @stable ICU 2.0
     */
    protected Transliterator(String ID, UnicodeFilter filter) {
        if (ID == null) {
            throw new NullPointerException();
        }
        this.ID = ID;
        setFilter(filter);
    }

    /**
     * Transliterates a segment of a string, with optional filtering.
     *
     * @param text the string to be transliterated
     * @param start the beginning index, inclusive; <code>0 &lt;= start
     * &lt;= limit</code>.
     * @param limit the ending index, exclusive; <code>start &lt;= limit
     * &lt;= text.length()</code>.
     * @return The new limit index. The text previously occupying <code>[start,
     * limit)</code> has been transliterated, possibly to a string of a different length, at <code>
     *     [start, </code><em>new-limit</em><code>)</code>, where <em>new-limit</em> is the return
     *     value. If the input offsets are out of bounds, the returned value is -1 and the input
     *     string remains unchanged.
     * @stable ICU 2.0
     */
    public final int transliterate(Replaceable text, int start, int limit) {
        if (start < 0 || limit < start || text.length() < limit) {
            return -1;
        }

        Position pos = new Position(start, limit, start);
        filteredTransliterate(text, pos, false, true);
        return pos.limit;
    }

    /**
     * Transliterates an entire string in place. Convenience method.
     *
     * @param text the string to be transliterated
     * @stable ICU 2.0
     */
    public final void transliterate(Replaceable text) {
        transliterate(text, 0, text.length());
    }

    /**
     * Transliterate an entire string and returns the result. Convenience method.
     *
     * @param text the string to be transliterated
     * @return The transliterated text
     * @stable ICU 2.0
     */
    public final String transliterate(String text) {
        ReplaceableString result = new ReplaceableString(text);
        transliterate(result);
        return result.toString();
    }

    /**
     * Transliterates the portion of the text buffer that can be transliterated unambiguosly after
     * new text has been inserted, typically as a result of a keyboard event. The new text in <code>
     * insertion</code> will be inserted into <code>text</code> at <code>index.contextLimit</code>,
     * advancing <code>index.contextLimit</code> by <code>insertion.length()</code>. Then the
     * transliterator will try to transliterate characters of <code>text</code> between <code>
     * index.start</code> and <code>index.contextLimit</code>. Characters before <code>index.start
     * </code> will not be changed.
     *
     * <p>Upon return, values in <code>index</code> will be updated. <code>index.contextStart</code>
     * will be advanced to the first character that future calls to this method will read. <code>
     * index.start</code> and <code>index.contextLimit</code> will be adjusted to delimit the range
     * of text that future calls to this method may change.
     *
     * <p>Typical usage of this method begins with an initial call with <code>index.contextStart
     * </code> and <code>index.contextLimit</code> set to indicate the portion of <code>text</code>
     * to be transliterated, and <code>index.start == index.contextStart</code>. Thereafter, <code>
     * index</code> can be used without modification in future calls, provided that all changes to
     * <code>text</code> are made via this method.
     *
     * <p>This method assumes that future calls may be made that will insert new text into the
     * buffer. As a result, it only performs unambiguous transliterations. After the last call to
     * this method, there may be untransliterated text that is waiting for more input to resolve an
     * ambiguity. In order to perform these pending transliterations, clients should call {@link
     * #finishTransliteration} after the last call to this method has been made.
     *
     * @param text the buffer holding transliterated and untransliterated text
     * @param index the start and limit of the text, the position of the cursor, and the start and
     *     limit of transliteration.
     * @param insertion text to be inserted and possibly transliterated into the translation buffer
     *     at <code>index.contextLimit</code>. If <code>null</code> then no text is inserted.
     * @see #handleTransliterate
     * @exception IllegalArgumentException if <code>index</code> is invalid
     * @stable ICU 2.0
     */
    public final void transliterate(Replaceable text, Position index, String insertion) {
        index.validate(text.length());

        //        int originalStart = index.contextStart;
        if (insertion != null) {
            text.replace(index.limit, index.limit, insertion);
            index.limit += insertion.length();
            index.contextLimit += insertion.length();
        }

        if (index.limit > 0 && UTF16.isLeadSurrogate(text.charAt(index.limit - 1))) {
            // Oops, there is a dangling lead surrogate in the buffer.
            // This will break most transliterators, since they will
            // assume it is part of a pair.  Don't transliterate until
            // more text comes in.
            return;
        }

        filteredTransliterate(text, index, true, true);

        // TODO
        // This doesn't work once we add quantifier support.  Need to rewrite
        // this code to support quantifiers and 'use maximum backup <n>;'.
        //
        //        index.contextStart = Math.max(index.start - getMaximumContextLength(),
        //                                      originalStart);
    }

    /**
     * Transliterates the portion of the text buffer that can be transliterated unambiguosly after a
     * new character has been inserted, typically as a result of a keyboard event. This is a
     * convenience method; see {@link #transliterate(Replaceable, Transliterator.Position, String)}
     * for details.
     *
     * @param text the buffer holding transliterated and untransliterated text
     * @param index the start and limit of the text, the position of the cursor, and the start and
     *     limit of transliteration.
     * @param insertion text to be inserted and possibly transliterated into the translation buffer
     *     at <code>index.contextLimit</code>.
     * @see #transliterate(Replaceable, Transliterator.Position, String)
     * @stable ICU 2.0
     */
    public final void transliterate(Replaceable text, Position index, int insertion) {
        transliterate(text, index, UTF16.valueOf(insertion));
    }

    /**
     * Transliterates the portion of the text buffer that can be transliterated unambiguosly. This
     * is a convenience method; see {@link #transliterate(Replaceable, Transliterator.Position,
     * String)} for details.
     *
     * @param text the buffer holding transliterated and untransliterated text
     * @param index the start and limit of the text, the position of the cursor, and the start and
     *     limit of transliteration.
     * @see #transliterate(Replaceable, Transliterator.Position, String)
     * @stable ICU 2.0
     */
    public final void transliterate(Replaceable text, Position index) {
        transliterate(text, index, null);
    }

    /**
     * Finishes any pending transliterations that were waiting for more characters. Clients should
     * call this method as the last call after a sequence of one or more calls to <code>
     * transliterate()</code>.
     *
     * @param text the buffer holding transliterated and untransliterated text.
     * @param index the array of indices previously passed to {@link #transliterate}
     * @stable ICU 2.0
     */
    public final void finishTransliteration(Replaceable text, Position index) {
        index.validate(text.length());
        filteredTransliterate(text, index, false, true);
    }

    /**
     * Abstract method that concrete subclasses define to implement their transliteration algorithm.
     * This method handles both incremental and non-incremental transliteration. Let <code>
     * originalStart</code> refer to the value of <code>pos.start</code> upon entry.
     *
     * <ul>
     *   <li>If <code>incremental</code> is false, then this method should transliterate all
     *       characters between <code>pos.start</code> and <code>pos.limit</code>. Upon return
     *       <code>pos.start</code> must == <code> pos.limit</code>.
     *   <li>If <code>incremental</code> is true, then this method should transliterate all
     *       characters between <code>pos.start</code> and <code>pos.limit</code> that can be
     *       unambiguously transliterated, regardless of future insertions of text at <code>
     *       pos.limit</code>. Upon return, <code>pos.start</code> should be in the range [<code>
     *       originalStart</code>, <code>pos.limit</code>). <code>pos.start</code> should be
     *       positioned such that characters [<code>originalStart</code>, <code>
     *  pos.start</code>) will not be changed in the future by this transliterator and characters [
     *       <code>pos.start</code>, <code>pos.limit</code>) are unchanged.
     * </ul>
     *
     * <p>Implementations of this method should also obey the following invariants:
     *
     * <ul>
     *   <li><code>pos.limit</code> and <code>pos.contextLimit</code> should be updated to reflect
     *       changes in length of the text between <code>pos.start</code> and <code>pos.limit</code>
     *       . The difference <code> pos.contextLimit - pos.limit</code> should not change.
     *   <li><code>pos.contextStart</code> should not change.
     *   <li>Upon return, neither <code>pos.start</code> nor <code>pos.limit</code> should be less
     *       than <code>originalStart</code>.
     *   <li>Text before <code>originalStart</code> and text after <code>pos.limit</code> should not
     *       change.
     *   <li>Text before <code>pos.contextStart</code> and text after <code> pos.contextLimit</code>
     *       should be ignored.
     * </ul>
     *
     * <p>Subclasses may safely assume that all characters in [<code>pos.start</code>, <code>
     * pos.limit</code>) are filtered. In other words, the filter has already been applied by the
     * time this method is called. See <code>filteredTransliterate()</code>.
     *
     * <p>This method is <b>not</b> for public consumption. Calling this method directly will
     * transliterate [<code>pos.start</code>, <code>pos.limit</code>) without applying the filter.
     * End user code should call <code>
     * transliterate()</code> instead of this method. Subclass code should call <code>
     * filteredTransliterate()</code> instead of this method.
     *
     * <p>
     *
     * @param text the buffer holding transliterated and untransliterated text
     * @param pos the indices indicating the start, limit, context start, and context limit of the
     *     text.
     * @param incremental if true, assume more text may be inserted at <code>pos.limit</code> and
     *     act accordingly. Otherwise, transliterate all text between <code>pos.start</code> and
     *     <code>pos.limit</code> and move <code>pos.start</code> up to <code>pos.limit</code>.
     * @see #transliterate
     * @stable ICU 2.0
     */
    protected abstract void handleTransliterate(
            Replaceable text, Position pos, boolean incremental);

    /**
     * Top-level transliteration method, handling filtering, incremental and non-incremental
     * transliteration, and rollback. All transliteration public API methods eventually call this
     * method with a rollback argument of true. Other entities may call this method but rollback
     * should be false.
     *
     * <p>If this transliterator has a filter, break up the input text into runs of unfiltered
     * characters. Pass each run to <subclass>.handleTransliterate().
     *
     * <p>In incremental mode, if rollback is true, perform a special incremental procedure in which
     * several passes are made over the input text, adding one character at a time, and committing
     * successful transliterations as they occur. Unsuccessful transliterations are rolled back and
     * retried with additional characters to give correct results.
     *
     * @param text the text to be transliterated
     * @param index the position indices
     * @param incremental if true, then assume more characters may be inserted at index.limit, and
     *     postpone processing to accommodate future incoming characters
     * @param rollback if true and if incremental is true, then perform special incremental
     *     processing, as described above, and undo partial transliterations where necessary. If
     *     incremental is false then this parameter is ignored.
     */
    private void filteredTransliterate(
            Replaceable text, Position index, boolean incremental, boolean rollback) {
        // Short circuit path for transliterators with no filter in
        // non-incremental mode.
        if (filter == null && !rollback) {
            handleTransliterate(text, index, incremental);
            return;
        }

        // ----------------------------------------------------------------------
        // This method processes text in two groupings:
        //
        // RUNS -- A run is a contiguous group of characters which are contained
        // in the filter for this transliterator (filter.contains(ch) == true).
        // Text outside of runs may appear as context but it is not modified.
        // The start and limit Position values are narrowed to each run.
        //
        // PASSES (incremental only) -- To make incremental mode work correctly,
        // each run is broken up into n passes, where n is the length (in code
        // points) of the run.  Each pass contains the first n characters.  If a
        // pass is completely transliterated, it is committed, and further passes
        // include characters after the committed text.  If a pass is blocked,
        // and does not transliterate completely, then this method rolls back
        // the changes made during the pass, extends the pass by one code point,
        // and tries again.
        // ----------------------------------------------------------------------

        // globalLimit is the limit value for the entire operation.  We
        // set index.limit to the end of each unfiltered run before
        // calling handleTransliterate(), so we need to maintain the real
        // value of index.limit here.  After each transliteration, we
        // update globalLimit for insertions or deletions that have
        // happened.
        int globalLimit = index.limit;

        // If there is a non-null filter, then break the input text up.  Say the
        // input text has the form:
        //   xxxabcxxdefxx
        // where 'x' represents a filtered character (filter.contains('x') ==
        // false).  Then we break this up into:
        //   xxxabc xxdef xx
        // Each pass through the loop consumes a run of filtered
        // characters (which are ignored) and a subsequent run of
        // unfiltered characters (which are transliterated).

        StringBuilder log = null;
        if (DEBUG) {
            log = new StringBuilder();
        }

        for (; ; ) {

            if (filter != null) {
                // Narrow the range to be transliterated to the first run
                // of unfiltered characters at or after index.start.

                // Advance past filtered chars
                int c;
                while (index.start < globalLimit
                        && !filter.contains(c = text.char32At(index.start))) {
                    index.start += UTF16.getCharCount(c);
                }

                // Find the end of this run of unfiltered chars
                index.limit = index.start;
                while (index.limit < globalLimit
                        && filter.contains(c = text.char32At(index.limit))) {
                    index.limit += UTF16.getCharCount(c);
                }
            }

            // Check to see if the unfiltered run is empty.  This only
            // happens at the end of the string when all the remaining
            // characters are filtered.
            if (index.start == index.limit) {
                break;
            }

            // Is this run incremental?  If there is additional
            // filtered text (if limit < globalLimit) then we pass in
            // an incremental value of false to force the subclass to
            // complete the transliteration for this run.
            boolean isIncrementalRun = (index.limit < globalLimit ? false : incremental);

            int delta;

            // Implement rollback.  To understand the need for rollback,
            // consider the following transliterator:
            //
            //  "t" is "a > A;"
            //  "u" is "A > b;"
            //  "v" is a compound of "t; NFD; u" with a filter [:Ll:]
            //
            // Now apply "v" to the input text "a".  The result is "b".  But if
            // the transliteration is done incrementally, then the NFD holds
            // things up after "t" has already transformed "a" to "A".  When
            // finishTransliterate() is called, "A" is _not_ processed because
            // it gets excluded by the [:Ll:] filter, and the end result is "A"
            // -- incorrect.  The problem is that the filter is applied to a
            // partially-transliterated result, when we only want it to apply to
            // input text.  Although this example describes a compound
            // transliterator containing NFD and a specific filter, it can
            // happen with any transliterator which does a partial
            // transformation in incremental mode into characters outside its
            // filter.
            //
            // To handle this, when in incremental mode we supply characters to
            // handleTransliterate() in several passes.  Each pass adds one more
            // input character to the input text.  That is, for input "ABCD", we
            // first try "A", then "AB", then "ABC", and finally "ABCD".  If at
            // any point we block (upon return, start < limit) then we roll
            // back.  If at any point we complete the run (upon return start ==
            // limit) then we commit that run.

            if (rollback && isIncrementalRun) {

                if (DEBUG) {
                    log.setLength(0);
                    System.out.println(
                            "filteredTransliterate{"
                                    + getID()
                                    + "}i: IN="
                                    + UtilityExtensions.formatInput(text, index));
                }

                int runStart = index.start;
                int runLimit = index.limit;
                int runLength = runLimit - runStart;

                // Make a rollback copy at the end of the string
                int rollbackOrigin = text.length();
                text.copy(runStart, runLimit, rollbackOrigin);

                // Variables reflecting the commitment of completely
                // transliterated text.  passStart is the runStart, advanced
                // past committed text.  rollbackStart is the rollbackOrigin,
                // advanced past rollback text that corresponds to committed
                // text.
                int passStart = runStart;
                int rollbackStart = rollbackOrigin;

                // The limit for each pass; we advance by one code point with
                // each iteration.
                int passLimit = index.start;

                // Total length, in 16-bit code units, of uncommitted text.
                // This is the length to be rolled back.
                int uncommittedLength = 0;

                // Total delta (change in length) for all passes
                int totalDelta = 0;

                // PASS MAIN LOOP -- Start with a single character, and extend
                // the text by one character at a time.  Roll back partial
                // transliterations and commit complete transliterations.
                for (; ; ) {
                    // Length of additional code point, either one or two
                    int charLength = UTF16.getCharCount(text.char32At(passLimit));
                    passLimit += charLength;
                    if (passLimit > runLimit) {
                        break;
                    }
                    uncommittedLength += charLength;

                    index.limit = passLimit;

                    if (DEBUG) {
                        log.setLength(0);
                        log.append("filteredTransliterate{" + getID() + "}i: ");
                        UtilityExtensions.formatInput(log, text, index);
                    }

                    // Delegate to subclass for actual transliteration.  Upon
                    // return, start will be updated to point after the
                    // transliterated text, and limit and contextLimit will be
                    // adjusted for length changes.
                    handleTransliterate(text, index, true);

                    if (DEBUG) {
                        log.append(" => ");
                        UtilityExtensions.formatInput(log, text, index);
                    }

                    delta = index.limit - passLimit; // change in length

                    // We failed to completely transliterate this pass.
                    // Roll back the text.  Indices remain unchanged; reset
                    // them where necessary.
                    if (index.start != index.limit) {
                        // Find the rollbackStart, adjusted for length changes
                        // and the deletion of partially transliterated text.
                        int rs = rollbackStart + delta - (index.limit - passStart);

                        // Delete the partially transliterated text
                        text.replace(passStart, index.limit, "");

                        // Copy the rollback text back
                        text.copy(rs, rs + uncommittedLength, passStart);

                        // Restore indices to their original values
                        index.start = passStart;
                        index.limit = passLimit;
                        index.contextLimit -= delta;

                        if (DEBUG) {
                            log.append(" (ROLLBACK)");
                        }
                    }

                    // We did completely transliterate this pass.  Update the
                    // commit indices to record how far we got.  Adjust indices
                    // for length change.
                    else {
                        // Move the pass indices past the committed text.
                        passStart = passLimit = index.start;

                        // Adjust the rollbackStart for length changes and move
                        // it past the committed text.  All characters we've
                        // processed to this point are committed now, so zero
                        // out the uncommittedLength.
                        rollbackStart += delta + uncommittedLength;
                        uncommittedLength = 0;

                        // Adjust indices for length changes.
                        runLimit += delta;
                        totalDelta += delta;
                    }

                    if (DEBUG) {
                        System.out.println(Utility.escape(log.toString()));
                    }
                }

                // Adjust overall limit and rollbackOrigin for insertions and
                // deletions.  Don't need to worry about contextLimit because
                // handleTransliterate() maintains that.
                rollbackOrigin += totalDelta;
                globalLimit += totalDelta;

                // Delete the rollback copy
                text.replace(rollbackOrigin, rollbackOrigin + runLength, "");

                // Move start past committed text
                index.start = passStart;
            } else {
                // Delegate to subclass for actual transliteration.
                if (DEBUG) {
                    log.setLength(0);
                    log.append("filteredTransliterate{" + getID() + "}: ");
                    UtilityExtensions.formatInput(log, text, index);
                }

                int limit = index.limit;
                handleTransliterate(text, index, isIncrementalRun);
                delta = index.limit - limit; // change in length

                if (DEBUG) {
                    log.append(" => ");
                    UtilityExtensions.formatInput(log, text, index);
                }

                // In a properly written transliterator, start == limit after
                // handleTransliterate() returns when incremental is false.
                // Catch cases where the subclass doesn't do this, and throw
                // an exception.  (Just pinning start to limit is a bad idea,
                // because what's probably happening is that the subclass
                // isn't transliterating all the way to the end, and it should
                // in non-incremental mode.)
                if (!isIncrementalRun && index.start != index.limit) {
                    throw new RuntimeException(
                            "ERROR: Incomplete non-incremental transliteration by " + getID());
                }

                // Adjust overall limit for insertions/deletions.  Don't need
                // to worry about contextLimit because handleTransliterate()
                // maintains that.
                globalLimit += delta;

                if (DEBUG) {
                    System.out.println(Utility.escape(log.toString()));
                }
            }

            if (filter == null || isIncrementalRun) {
                break;
            }

            // If we did completely transliterate this
            // run, then repeat with the next unfiltered run.
        }

        // Start is valid where it is.  Limit needs to be put back where
        // it was, modulo adjustments for deletions/insertions.
        index.limit = globalLimit;

        if (DEBUG) {
            System.out.println(
                    "filteredTransliterate{"
                            + getID()
                            + "}: OUT="
                            + UtilityExtensions.formatInput(text, index));
        }
    }

    /**
     * Transliterate a substring of text, as specified by index, taking filters into account. This
     * method is for subclasses that need to delegate to another transliterator.
     *
     * @param text the text to be transliterated
     * @param index the position indices
     * @param incremental if true, then assume more characters may be inserted at index.limit, and
     *     postpone processing to accommodate future incoming characters
     * @stable ICU 2.0
     */
    public void filteredTransliterate(Replaceable text, Position index, boolean incremental) {
        filteredTransliterate(text, index, incremental, false);
    }

    /**
     * Returns the length of the longest context required by this transliterator. This is
     * <em>preceding</em> context. The default value is zero, but subclasses can change this by
     * calling <code>setMaximumContextLength()</code>. For example, if a transliterator translates
     * "ddd" (where d is any digit) to "555" when preceded by "(ddd)", then the preceding context
     * length is 5, the length of "(ddd)".
     *
     * @return The maximum number of preceding context characters this transliterator needs to
     *     examine
     * @stable ICU 2.0
     */
    public final int getMaximumContextLength() {
        return maximumContextLength;
    }

    /**
     * Method for subclasses to use to set the maximum context length.
     *
     * @see #getMaximumContextLength
     * @stable ICU 2.0
     */
    protected void setMaximumContextLength(int a) {
        if (a < 0) {
            throw new IllegalArgumentException("Invalid context length " + a);
        }
        maximumContextLength = a;
    }

    /**
     * Returns a programmatic identifier for this transliterator. If this identifier is passed to
     * <code>getInstance()</code>, it will return this object, if it has been registered.
     *
     * @see #registerClass
     * @see #getAvailableIDs
     * @stable ICU 2.0
     */
    public final String getID() {
        return ID;
    }

    /**
     * Set the programmatic identifier for this transliterator. Only for use by subclasses.
     *
     * @stable ICU 2.0
     */
    protected final void setID(String id) {
        ID = id;
    }

    /**
     * Returns a name for this transliterator that is appropriate for display to the user in the
     * default <code>DISPLAY</code> locale. See {@link #getDisplayName(String,Locale)} for details.
     *
     * @see com.ibm.icu.util.ULocale.Category#DISPLAY
     * @stable ICU 2.0
     */
    public static final String getDisplayName(String ID) {
        return getDisplayName(ID, ULocale.getDefault(Category.DISPLAY));
    }

    /**
     * Returns a name for this transliterator that is appropriate for display to the user in the
     * given locale. This name is taken from the locale resource data in the standard manner of the
     * <code>java.text</code> package.
     *
     * <p>If no localized names exist in the system resource bundles, a name is synthesized using a
     * localized <code>MessageFormat</code> pattern from the resource data. The arguments to this
     * pattern are an integer followed by one or two strings. The integer is the number of strings,
     * either 1 or 2. The strings are formed by splitting the ID for this transliterator at the
     * first '-'. If there is no '-', then the entire ID forms the only string.
     *
     * @param inLocale the Locale in which the display name should be localized.
     * @see java.text.MessageFormat
     * @stable ICU 2.0
     */
    public static String getDisplayName(String id, Locale inLocale) {
        return getDisplayName(id, ULocale.forLocale(inLocale));
    }

    /**
     * Returns a name for this transliterator that is appropriate for display to the user in the
     * given locale. This name is taken from the locale resource data in the standard manner of the
     * <code>java.text</code> package.
     *
     * <p>If no localized names exist in the system resource bundles, a name is synthesized using a
     * localized <code>MessageFormat</code> pattern from the resource data. The arguments to this
     * pattern are an integer followed by one or two strings. The integer is the number of strings,
     * either 1 or 2. The strings are formed by splitting the ID for this transliterator at the
     * first '-'. If there is no '-', then the entire ID forms the only string.
     *
     * @param inLocale the ULocale in which the display name should be localized.
     * @see java.text.MessageFormat
     * @stable ICU 3.2
     */
    public static String getDisplayName(String id, ULocale inLocale) {

        // Resource bundle containing display name keys and the
        // RB_RULE_BASED_IDS array.
        //
        // If we ever integrate this with the Sun JDK, the resource bundle
        // root will change to sun.text.resources.LocaleElements

        ICUResourceBundle bundle =
                (ICUResourceBundle)
                        UResourceBundle.getBundleInstance(ICUData.ICU_TRANSLIT_BASE_NAME, inLocale);

        // Normalize the ID
        String stv[] = TransliteratorIDParser.IDtoSTV(id);
        if (stv == null) {
            // No target; malformed id
            return "";
        }
        String ID = stv[0] + '-' + stv[1];
        if (stv[2] != null && stv[2].length() > 0) {
            ID = ID + '/' + stv[2];
        }

        // Use the registered display name, if any
        String n = displayNameCache.get(new CaseInsensitiveString(ID));
        if (n != null) {
            return n;
        }

        // Use display name for the entire transliterator, if it
        // exists.
        try {
            return bundle.getString(RB_DISPLAY_NAME_PREFIX + ID);
        } catch (MissingResourceException e) {
        }

        try {
            // Construct the formatter first; if getString() fails
            // we'll exit the try block
            MessageFormat format = new MessageFormat(bundle.getString(RB_DISPLAY_NAME_PATTERN));
            // Construct the argument array
            Object[] args = new Object[] {2, stv[0], stv[1]};

            // Use display names for the scripts, if they exist
            for (int j = 1; j <= 2; ++j) {
                try {
                    args[j] = bundle.getString(RB_SCRIPT_DISPLAY_NAME_PREFIX + (String) args[j]);
                } catch (MissingResourceException e) {
                }
            }

            // Format it using the pattern in the resource
            return (stv[2].length() > 0)
                    ? (format.format(args) + '/' + stv[2])
                    : format.format(args);
        } catch (MissingResourceException e2) {
        }

        // We should not reach this point unless there is something
        // wrong with the build or the RB_DISPLAY_NAME_PATTERN has
        // been deleted from the root RB_LOCALE_ELEMENTS resource.
        throw new RuntimeException();
    }

    /**
     * Returns the filter used by this transliterator, or {@code null} if this transliterator uses
     * no filter.
     *
     * @stable ICU 2.0
     */
    public final UnicodeFilter getFilter() {
        return filter;
    }

    /**
     * Changes the filter used by this transliterator. If the filter is set to {@code null} then no
     * filtering will occur.
     *
     * <p>Callers must take care if a transliterator is in use by multiple threads. The filter
     * should not be changed by one thread while another thread may be transliterating.
     *
     * @stable ICU 2.0
     */
    public void setFilter(UnicodeFilter filter) {
        if (filter == null) {
            this.filter = null;
        } else {
            try {
                // fast high-runner case
                this.filter = new UnicodeSet((UnicodeSet) filter).freeze();
            } catch (Exception e) {
                this.filter = new UnicodeSet();
                filter.addMatchSetTo(this.filter);
                this.filter.freeze();
            }
        }
    }

    /**
     * Returns a <code>Transliterator</code> object given its ID. The ID must be either a system
     * transliterator ID or a ID registered using <code>registerClass()</code>.
     *
     * @param ID a valid ID, as enumerated by <code>getAvailableIDs()</code>
     * @return A <code>Transliterator</code> object with the given ID
     * @exception IllegalArgumentException if the given ID is invalid.
     * @stable ICU 2.0
     */
    public static final Transliterator getInstance(String ID) {
        return getInstance(ID, FORWARD);
    }

    /**
     * Returns a <code>Transliterator</code> object given its ID. The ID must be either a system
     * transliterator ID or a ID registered using <code>registerClass()</code>.
     *
     * @param ID a valid ID, as enumerated by <code>getAvailableIDs()</code>
     * @param dir either FORWARD or REVERSE. If REVERSE then the inverse of the given ID is
     *     instantiated.
     * @return A <code>Transliterator</code> object with the given ID
     * @exception IllegalArgumentException if the given ID is invalid.
     * @see #registerClass
     * @see #getAvailableIDs
     * @see #getID
     * @stable ICU 2.0
     */
    public static Transliterator getInstance(String ID, int dir) {
        StringBuilder canonID = new StringBuilder();
        List<SingleID> list = new ArrayList<>();
        UnicodeSet[] globalFilter = new UnicodeSet[1];
        if (!TransliteratorIDParser.parseCompoundID(ID, dir, canonID, list, globalFilter)) {
            throw new IllegalArgumentException("Invalid ID " + ID);
        }

        List<Transliterator> translits = TransliteratorIDParser.instantiateList(list);

        // assert(list.size() > 0);
        Transliterator t = null;
        if (list.size() > 1 || canonID.indexOf(";") >= 0) {
            // [NOTE: If it's a compoundID, we instantiate a CompoundTransliterator even if it only
            // has one child transliterator.  This is so that toRules() will return the right thing
            // (without any inactive ID), but our main ID still comes out correct.  That is, if we
            // instantiate "(Lower);Latin-Greek;", we want the rules to come out as "::Latin-Greek;"
            // even though the ID is "(Lower);Latin-Greek;".
            t = new CompoundTransliterator(translits);
        } else {
            t = translits.get(0);
        }

        t.setID(canonID.toString());
        if (globalFilter[0] != null) {
            t.setFilter(globalFilter[0]);
        }
        return t;
    }

    /**
     * Create a transliterator from a basic ID. This is an ID containing only the forward direction
     * source, target, and variant.
     *
     * @param id a basic ID of the form S-T or S-T/V.
     * @param canonID canonical ID to apply to the result, or null to leave the ID unchanged
     * @return a newly created Transliterator or null if the ID is invalid.
     */
    static Transliterator getBasicInstance(String id, String canonID) {
        StringBuilder s = new StringBuilder();
        Transliterator t = registry.get(id, s);
        if (s.length() != 0) {
            // assert(t==0);
            // Instantiate an alias
            t = getInstance(s.toString(), FORWARD);
        }
        if (t != null && canonID != null) {
            t.setID(canonID);
        }
        return t;
    }

    /**
     * Returns a <code>Transliterator</code> object constructed from the given rule string. This
     * will be a rule-based Transliterator, if the rule string contains only rules, or a compound
     * Transliterator, if it contains ID blocks, or a null Transliterator, if it contains ID blocks
     * which parse as empty for the given direction.
     *
     * @param ID the id for the transliterator.
     * @param rules rules, separated by ';'
     * @param dir either FORWARD or REVERSE.
     * @return a newly created Transliterator
     * @throws IllegalArgumentException if there is a problem with the ID or the rules
     * @stable ICU 2.0
     */
    public static final Transliterator createFromRules(String ID, String rules, int dir) {
        Transliterator t = null;

        TransliteratorParser parser = new TransliteratorParser();
        parser.parse(rules, dir);

        // NOTE: The logic here matches that in TransliteratorRegistry.
        if (parser.idBlockVector.size() == 0 && parser.dataVector.size() == 0) {
            t = new NullTransliterator();
        } else if (parser.idBlockVector.size() == 0 && parser.dataVector.size() == 1) {
            t = new RuleBasedTransliterator(ID, parser.dataVector.get(0), parser.compoundFilter);
        } else if (parser.idBlockVector.size() == 1 && parser.dataVector.size() == 0) {
            // idBlock, no data -- this is an alias.  The ID has
            // been munged from reverse into forward mode, if
            // necessary, so instantiate the ID in the forward
            // direction.
            if (parser.compoundFilter != null) {
                t =
                        getInstance(
                                parser.compoundFilter.toPattern(false)
                                        + ";"
                                        + parser.idBlockVector.get(0));
            } else {
                t = getInstance(parser.idBlockVector.get(0));
            }

            if (t != null) {
                t.setID(ID);
            }
        } else {
            List<Transliterator> transliterators = new ArrayList<>();
            int passNumber = 1;

            int limit = Math.max(parser.idBlockVector.size(), parser.dataVector.size());
            for (int i = 0; i < limit; i++) {
                if (i < parser.idBlockVector.size()) {
                    String idBlock = parser.idBlockVector.get(i);
                    if (idBlock.length() > 0) {
                        Transliterator temp = getInstance(idBlock);
                        if (!(temp instanceof NullTransliterator))
                            transliterators.add(getInstance(idBlock));
                    }
                }
                if (i < parser.dataVector.size()) {
                    Data data = parser.dataVector.get(i);
                    transliterators.add(
                            new RuleBasedTransliterator("%Pass" + passNumber++, data, null));
                }
            }

            t = new CompoundTransliterator(transliterators, passNumber - 1);
            t.setID(ID);
            if (parser.compoundFilter != null) {
                t.setFilter(parser.compoundFilter);
            }
        }

        return t;
    }

    /**
     * Returns a rule string for this transliterator.
     *
     * @param escapeUnprintable if true, then unprintable characters will be converted to escape
     *     form backslash-'u' or backslash-'U'.
     * @stable ICU 2.0
     */
    public String toRules(boolean escapeUnprintable) {
        return baseToRules(escapeUnprintable);
    }

    /**
     * Returns a rule string for this transliterator. This is a non-overrideable base class
     * implementation that subclasses may call. It simply munges the ID into the correct format,
     * that is, "foo" =&gt; "::foo".
     *
     * @param escapeUnprintable if true, then unprintable characters will be converted to escape
     *     form backslash-'u' or backslash-'U'.
     * @stable ICU 2.0
     */
    protected final String baseToRules(boolean escapeUnprintable) {
        // The base class implementation of toRules munges the ID into
        // the correct format.  That is: foo => ::foo
        // KEEP in sync with rbt_pars
        if (escapeUnprintable) {
            StringBuilder rulesSource = new StringBuilder();
            String id = getID();
            for (int i = 0; i < id.length(); ) {
                int c = UTF16.charAt(id, i);
                if (!Utility.escapeUnprintable(rulesSource, c)) {
                    rulesSource.appendCodePoint(c);
                }
                i += UTF16.getCharCount(c);
            }
            rulesSource.insert(0, "::");
            rulesSource.append(ID_DELIM);
            return rulesSource.toString();
        }
        return "::" + getID() + ID_DELIM;
    }

    /**
     * Return the elements that make up this transliterator. For example, if the transliterator
     * "NFD;Jamo-Latin;Latin-Greek" were created, the return value of this method would be an array
     * of the three transliterator objects that make up that transliterator: [NFD, Jamo-Latin,
     * Latin-Greek].
     *
     * <p>If this transliterator is not composed of other transliterators, then this method will
     * return an array of length one containing a reference to this transliterator.
     *
     * @return an array of one or more transliterators that make up this transliterator
     * @stable ICU 3.0
     */
    public Transliterator[] getElements() {
        Transliterator result[];
        if (this instanceof CompoundTransliterator) {
            CompoundTransliterator cpd = (CompoundTransliterator) this;
            result = new Transliterator[cpd.getCount()];
            for (int i = 0; i < result.length; ++i) {
                result[i] = cpd.getTransliterator(i);
            }
        } else {
            result = new Transliterator[] {this};
        }
        return result;
    }

    /**
     * Returns the set of all characters that may be modified in the input text by this
     * Transliterator. This incorporates this object's current filter; if the filter is changed, the
     * return value of this function will change. The default implementation returns an empty set.
     * Some subclasses may override {@link #handleGetSourceSet} to return a more precise result. The
     * return result is approximate in any case and is intended for use by tests, tools, or
     * utilities.
     *
     * @see #getTargetSet
     * @see #handleGetSourceSet
     * @stable ICU 2.2
     */
    public final UnicodeSet getSourceSet() {
        UnicodeSet result = new UnicodeSet();
        addSourceTargetSet(
                getFilterAsUnicodeSet(UnicodeSet.ALL_CODE_POINTS), result, new UnicodeSet());
        return result;
    }

    /**
     * Framework method that returns the set of all characters that may be modified in the input
     * text by this Transliterator, ignoring the effect of this object's filter. The base class
     * implementation returns the empty set. Subclasses that wish to implement this should override
     * this method.
     *
     * @return the set of characters that this transliterator may modify. The set may be modified,
     *     so subclasses should return a newly-created object.
     * @see #getSourceSet
     * @see #getTargetSet
     * @stable ICU 2.2
     */
    protected UnicodeSet handleGetSourceSet() {
        return new UnicodeSet();
    }

    /**
     * Returns the set of all characters that may be generated as replacement text by this
     * transliterator. The default implementation returns the empty set. Some subclasses may
     * override this method to return a more precise result. The return result is approximate in any
     * case and is intended for use by tests, tools, or utilities requiring such meta-information.
     *
     * <p>Warning. You might expect an empty filter to always produce an empty target. However,
     * consider the following:
     *
     * <pre>
     * [Pp]{}[\u03A3\u03C2\u03C3\u03F7\u03F8\u03FA\u03FB] &gt; \';
     * </pre>
     *
     * With a filter of [], you still get some elements in the target set, because this rule will
     * still match. It could be recast to the following if it were important.
     *
     * <pre>
     * [Pp]{([\u03A3\u03C2\u03C3\u03F7\u03F8\u03FA\u03FB])} &gt; \' | $1;
     * </pre>
     *
     * @see #getTargetSet
     * @stable ICU 2.2
     */
    public UnicodeSet getTargetSet() {
        UnicodeSet result = new UnicodeSet();
        addSourceTargetSet(
                getFilterAsUnicodeSet(UnicodeSet.ALL_CODE_POINTS), new UnicodeSet(), result);
        return result;
    }

    /**
     * Returns the set of all characters that may be generated as replacement text by this
     * transliterator, filtered by BOTH the input filter, and the current getFilter().
     *
     * <p>SHOULD BE OVERRIDDEN BY SUBCLASSES. It is probably an error for any transliterator to NOT
     * override this, but we can't force them to for backwards compatibility.
     *
     * <p>Other methods vector through this.
     *
     * <p>When gathering the information on source and target, the compound transliterator makes
     * things complicated. For example, suppose we have:
     *
     * <pre>
     * Global FILTER = [ax]
     * a &gt; b;
     * :: NULL;
     * b &gt; c;
     * x &gt; d;
     * </pre>
     *
     * While the filter just allows a and x, b is an intermediate result, which could produce c. So
     * the source and target sets cannot be gathered independently. What we have to do is filter the
     * sources for the first transliterator according to the global filter, intersect that
     * transliterator's filter. Based on that we get the target. The next transliterator gets as a
     * global filter (global + last target). And so on.
     *
     * <p>There is another complication:
     *
     * <pre>
     * Global FILTER = [ax]
     * a &gt;|b;
     * b &gt;c;
     * </pre>
     *
     * Even though b would be filtered from the input, whenever we have a backup, it could be part
     * of the input. So ideally we will change the global filter as we go.
     *
     * @param targetSet TODO
     * @see #getTargetSet
     * @internal
     * @deprecated This API is ICU internal only.
     */
    @Deprecated
    public void addSourceTargetSet(
            UnicodeSet inputFilter, UnicodeSet sourceSet, UnicodeSet targetSet) {
        UnicodeSet myFilter = getFilterAsUnicodeSet(inputFilter);
        UnicodeSet temp = new UnicodeSet(handleGetSourceSet()).retainAll(myFilter);
        // use old method, if we don't have anything better
        sourceSet.addAll(temp);
        // clumsy guess with target
        for (String s : temp) {
            String t = transliterate(s);
            if (!s.equals(t)) {
                targetSet.addAll(t);
            }
        }
    }

    /**
     * Returns the intersectionof this instance's filter intersected with an external filter. The
     * externalFilter must be frozen (it is frozen if not). The result may be frozen, so don't
     * attempt to modify.
     *
     * @internal
     * @deprecated This API is ICU internal only.
     */
    @Deprecated
    // TODO change to getMergedFilter
    public UnicodeSet getFilterAsUnicodeSet(UnicodeSet externalFilter) {
        if (filter == null) {
            return externalFilter;
        }
        UnicodeSet filterSet = new UnicodeSet(externalFilter);
        // Most, but not all filters will be UnicodeSets.  Optimize for
        // the high-runner case.
        UnicodeSet temp;
        try {
            temp = filter;
        } catch (ClassCastException e) {
            filter.addMatchSetTo(temp = new UnicodeSet());
        }
        return filterSet.retainAll(temp).freeze();
    }

    /**
     * Returns this transliterator's inverse. See the class documentation for details. This
     * implementation simply inverts the two entities in the ID and attempts to retrieve the
     * resulting transliterator. That is, if <code>getID()</code> returns "A-B", then this method
     * will return the result of <code>getInstance("B-A")</code>, or <code>null</code> if that call
     * fails.
     *
     * <p>Subclasses with knowledge of their inverse may wish to override this method.
     *
     * @return a transliterator that is an inverse, not necessarily exact, of this transliterator,
     *     or <code>null</code> if no such transliterator is registered.
     * @see #registerClass
     * @stable ICU 2.0
     */
    public final Transliterator getInverse() {
        return getInstance(ID, REVERSE);
    }

    /**
     * Registers a subclass of <code>Transliterator</code> with the system. This subclass must have
     * a public constructor taking no arguments. When that constructor is called, the resulting
     * object must return the <code>ID</code> passed to this method if its <code>getID()</code>
     * method is called.
     *
     * @param ID the result of <code>getID()</code> for this transliterator
     * @param transClass a subclass of <code>Transliterator</code>
     * @see #unregister
     * @stable ICU 2.0
     */
    public static void registerClass(
            String ID, Class<? extends Transliterator> transClass, String displayName) {
        registry.put(ID, transClass, true);
        if (displayName != null) {
            displayNameCache.put(new CaseInsensitiveString(ID), displayName);
        }
    }

    /**
     * Register a factory object with the given ID. The factory method should return a new instance
     * of the given transliterator.
     *
     * <p>Because ICU may choose to cache Transliterator objects internally, this must be called at
     * application startup, prior to any calls to Transliterator.getInstance to avoid undefined
     * behavior.
     *
     * @param ID the ID of this transliterator
     * @param factory the factory object
     * @stable ICU 2.0
     */
    public static void registerFactory(String ID, Factory factory) {
        registry.put(ID, factory, true);
    }

    /**
     * Register a Transliterator object with the given ID.
     *
     * <p>Because ICU may choose to cache Transliterator objects internally, this must be called at
     * application startup, prior to any calls to Transliterator.getInstance to avoid undefined
     * behavior.
     *
     * @param trans the Transliterator object
     * @stable ICU 2.2
     */
    public static void registerInstance(Transliterator trans) {
        registry.put(trans.getID(), trans, true);
    }

    /**
     * Register a Transliterator object.
     *
     * <p>Because ICU may choose to cache Transliterator objects internally, this must be called at
     * application startup, prior to any calls to Transliterator.getInstance to avoid undefined
     * behavior.
     *
     * @param trans the Transliterator object
     */
    static void registerInstance(Transliterator trans, boolean visible) {
        registry.put(trans.getID(), trans, visible);
    }

    /**
     * Register an ID as an alias of another ID. Instantiating alias ID produces the same result as
     * instantiating the original ID. This is generally used to create short aliases of compound
     * IDs.
     *
     * <p>Because ICU may choose to cache Transliterator objects internally, this must be called at
     * application startup, prior to any calls to Transliterator.getInstance to avoid undefined
     * behavior.
     *
     * @param aliasID The new ID being registered.
     * @param realID The existing ID that the new ID should be an alias of.
     * @stable ICU 3.6
     */
    public static void registerAlias(String aliasID, String realID) {
        registry.put(aliasID, realID, true);
    }

    /**
     * Register two targets as being inverses of one another. For example, calling
     * registerSpecialInverse("NFC", "NFD", true) causes Transliterator to form the following
     * inverse relationships:
     *
     * <pre>NFC =&gt; NFD
     * Any-NFC =&gt; Any-NFD
     * NFD =&gt; NFC
     * Any-NFD =&gt; Any-NFC</pre>
     *
     * (Without the special inverse registration, the inverse of NFC would be NFC-Any.) Note that
     * NFD is shorthand for Any-NFD, but that the presence or absence of "Any-" is preserved.
     *
     * <p>The relationship is symmetrical; registering (a, b) is equivalent to registering (b, a).
     *
     * <p>The relevant IDs must still be registered separately as factories or classes.
     *
     * <p>Only the targets are specified. Special inverses always have the form Any-Target1
     * &lt;=&gt; Any-Target2. The target should have canonical casing (the casing desired to be
     * produced when an inverse is formed) and should contain no whitespace or other extraneous
     * characters.
     *
     * @param target the target against which to register the inverse
     * @param inverseTarget the inverse of target, that is Any-target.getInverse() =&gt;
     *     Any-inverseTarget
     * @param bidirectional if true, register the reverse relation as well, that is,
     *     Any-inverseTarget.getInverse() =&gt; Any-target
     */
    static void registerSpecialInverse(String target, String inverseTarget, boolean bidirectional) {
        TransliteratorIDParser.registerSpecialInverse(target, inverseTarget, bidirectional);
    }

    /**
     * Unregisters a transliterator or class. This may be either a system transliterator or a user
     * transliterator or class.
     *
     * @param ID the ID of the transliterator or class
     * @see #registerClass
     * @stable ICU 2.0
     */
    public static void unregister(String ID) {
        displayNameCache.remove(new CaseInsensitiveString(ID));
        registry.remove(ID);
    }

    /**
     * Returns an enumeration over the programmatic names of registered <code>Transliterator</code>
     * objects. This includes both system transliterators and user transliterators registered using
     * <code>registerClass()</code>. The enumerated names may be passed to <code>getInstance()
     * </code>.
     *
     * @return An <code>Enumeration</code> over <code>String</code> objects
     * @see #getInstance
     * @see #registerClass
     * @stable ICU 2.0
     */
    public static final Enumeration<String> getAvailableIDs() {
        return registry.getAvailableIDs();
    }

    /**
     * Returns an enumeration over the source names of registered transliterators. Source names may
     * be passed to getAvailableTargets() to obtain available targets for each source.
     *
     * @stable ICU 2.0
     */
    public static final Enumeration<String> getAvailableSources() {
        return registry.getAvailableSources();
    }

    /**
     * Returns an enumeration over the target names of registered transliterators having a given
     * source name. Target names may be passed to getAvailableVariants() to obtain available
     * variants for each source and target pair.
     *
     * @stable ICU 2.0
     */
    public static final Enumeration<String> getAvailableTargets(String source) {
        return registry.getAvailableTargets(source);
    }

    /**
     * Returns an enumeration over the variant names of registered transliterators having a given
     * source name and target name.
     *
     * @stable ICU 2.0
     */
    public static final Enumeration<String> getAvailableVariants(String source, String target) {
        return registry.getAvailableVariants(source, target);
    }

    private static final String ROOT = "root", RB_RULE_BASED_IDS = "RuleBasedTransliteratorIDs";

    static {
        registry = new TransliteratorRegistry();

        // The display name cache starts out empty
        displayNameCache =
                Collections.synchronizedMap(new HashMap<CaseInsensitiveString, String>());
        /* The following code parses the index table located in
         * icu/data/translit/root.txt.  The index is an n x 4 table
         * that follows this format:
         *  <id>{
         *      file{
         *          resource{"<resource>"}
         *          direction{"<direction>"}
         *      }
         *  }
         *  <id>{
         *      internal{
         *          resource{"<resource>"}
         *          direction{"<direction"}
         *       }
         *  }
         *  <id>{
         *      alias{"<getInstanceArg"}
         *  }
         * <id> is the ID of the system transliterator being defined.  These
         * are public IDs enumerated by Transliterator.getAvailableIDs(),
         * unless the second field is "internal".
         *
         * <resource> is a ResourceReader resource name.  Currently these refer
         * to file names under com/ibm/text/resources.  This string is passed
         * directly to ResourceReader, together with <encoding>.
         *
         * <direction> is either "FORWARD" or "REVERSE".
         *
         * <getInstanceArg> is a string to be passed directly to
         * Transliterator.getInstance().  The returned Transliterator object
         * then has its ID changed to <id> and is returned.
         *
         * The extra blank field on "alias" lines is to make the array square.
         */
        UResourceBundle bundle, transIDs, colBund;
        bundle = UResourceBundle.getBundleInstance(ICUData.ICU_TRANSLIT_BASE_NAME, ROOT);
        transIDs = bundle.get(RB_RULE_BASED_IDS);

        int row, maxRows;
        maxRows = transIDs.getSize();
        for (row = 0; row < maxRows; row++) {
            colBund = transIDs.get(row);
            String ID = colBund.getKey();
            if (ID.indexOf("-t-") >= 0) {
                continue;
            }
            UResourceBundle res = colBund.get(0);
            String type = res.getKey();
            if (type.equals("file") || type.equals("internal")) {
                // Rest of line is <resource>:<encoding>:<direction>
                //                pos       colon      c2
                int rowIndex = row;
                Supplier<String> resSupplier =
                        () -> {
                            // Capture the row Id instead of the UResourceBundle object
                            // due to the memory cost.
                            UResourceBundle rootBund =
                                    UResourceBundle.getBundleInstance(
                                            ICUData.ICU_TRANSLIT_BASE_NAME, ROOT);
                            UResourceBundle transIDsBund = rootBund.get(RB_RULE_BASED_IDS);
                            UResourceBundle thisBund = transIDsBund.get(rowIndex).get(0);
                            return thisBund.getString("resource");
                        };
                int dir;
                String direction = res.getString("direction");
                switch (direction.charAt(0)) {
                    case 'F':
                        dir = FORWARD;
                        break;
                    case 'R':
                        dir = REVERSE;
                        break;
                    default:
                        throw new RuntimeException("Can't parse direction: " + direction);
                }
                registry.put(
                        ID,
                        resSupplier, // resource
                        dir,
                        !type.equals("internal"));
            } else if (type.equals("alias")) {
                // 'alias'; row[2]=createInstance argument
                String resString = res.getString();
                registry.put(ID, resString, true);
            } else {
                // Unknown type
                throw new RuntimeException("Unknown type: " + type);
            }
        }

        registerSpecialInverse(NullTransliterator.SHORT_ID, NullTransliterator.SHORT_ID, false);

        // Register non-rule-based transliterators
        registerClass(NullTransliterator._ID, NullTransliterator.class, null);
        RemoveTransliterator.register();
        EscapeTransliterator.register();
        UnescapeTransliterator.register();
        LowercaseTransliterator.register();
        UppercaseTransliterator.register();
        TitlecaseTransliterator.register();
        CaseFoldTransliterator.register();
        UnicodeNameTransliterator.register();
        NameUnicodeTransliterator.register();
        NormalizationTransliterator.register();
        BreakTransliterator.register();
        AnyTransliterator.register(); // do this last!
    }

    /**
     * Register the script-based "Any" transliterators: Any-Latin, Any-Greek
     *
     * @internal
     * @deprecated This API is ICU internal only.
     */
    @Deprecated
    public static void registerAny() {
        AnyTransliterator.register();
    }

    /**
     * The factory interface for transliterators. Transliterator subclasses can register factory
     * objects for IDs using the registerFactory() method of Transliterator. When invoked, the
     * factory object will be passed the ID being instantiated. This makes it possible to register
     * one factory method to more than one ID, or for a factory method to parameterize its result
     * based on the variant.
     *
     * @stable ICU 2.0
     */
    public static interface Factory {
        /**
         * Return a transliterator for the given ID.
         *
         * @stable ICU 2.0
         */
        Transliterator getInstance(String ID);
    }

    /**
     * Implements StringTransform via this method.
     *
     * @param source text to be transformed (eg lowercased)
     * @return result
     * @stable ICU 3.8
     */
    @Override
    public String transform(String source) {
        return transliterate(source);
    }
}
